Conventional aluminum ladders can be folded for storage or opened for use. Referring to FIG. 11, it can be seen that a conventional aluminum ladder can be propped open at a certain angle only, that is, the ladder is only adapted for one fixed working condition. Therefore the shape of the ladder can not be changed and so in practice has limited usage. A conventional ladder, for example, can not be folded into the shape shown in FIG. 10.